This invention relates to speed change mechanisms for changing gear speed ratios between positions selectable by an operator. More particularly, this invention relates to torque limiting devices within speed change mechanisms of the type actuated by a travelling nut on a screw shaft. Screw shafts utilized in such structures have endured high fatigue failure rates as a result of cyclic torque loading of the screw shaft created via nut impact stoppage at the screw ends. Some solutions to the cyclic torque loading problem have been tried, as for example, snubber structures having non-metallic resilient thrust absorbing means as disclosed in U.S. Pat. Nos. 2,398,841 and 2,424,492, both to Morris. These, however, have suffered short-life problems. The rubber or "neoprene" deformable elements as utilized become crazed and hardened after successive impacts, and are thus susceptible to early failure. U.S. Pat. No. 2,738,685 to Hartz discloses the use of a helical spring to absorb shock at the ends of nut travel along a screw by unwinding the spring to expand same into frictional sliding engagement with the inner surface of a torque tube.